Some packaged power semiconductor device die in operation utilize vertical current flow through the semiconductor substrate (e.g., silicon) to a drain (or source) contact on the bottom side of the die and then through the die pad which can be exposed for electrical contact and enhanced cooling under the semiconductor substrate. The current path can also be from a non-exposed die pad out to a substrate pin, such as using a bond wire from the die pad to the substrate pin. There can be other ways to bring an electrically conductive path from the die pad to the outside of the package, such as bond wires or fusing some leads to the die pad itself. For example, the vertical power semiconductor device die can comprise a metal-oxide-semiconductor field-effect transistor (MOSFET) generally comprising either a planar gate or a trench gate, a vertical bipolar device, or an insulated-gate bipolar transistor (IGBT).
The die attach material being in series in a current flow path of the device is important in this power device arrangement and thus needs to be electrically conductive. Due to material and process limitations, this electrically conductive die attach material, such as solder or metal (e.g., silver) filled epoxy, or a sintered material (e.g., a silver sintered material or a copper sintered material) can crack or delaminate, including during reliability testing which involves thermal cycling.
Thermal cycling can involve the device subjected to alternating environments of high temperature (e.g., 150° C.) and cold temperature (−65° C.), and can also go through power cycling, where the device is allowed to be turned on and off repeatedly. In the case of a packaged power MOSFET, damage to the conductive die attach material can cause an increase in the both the device's ON resistance (RDSon) and the conductive die attach materials thermal resistance. Such an RDSon increase can cause higher power dissipation, resulting in temperatures above allowable reliable limits, and can eventually cause an electrical failure of the device.